Variable-speed generators or, generally speaking, generators having a frequency that diverges from the grid frequency can be connected to the electrical power grid by converters that adapt the voltage and the frequency being generated by the generator to the voltage and the frequency of the electrical power grid. Various devices are used as converters for this purpose such as, for instance, so-called direct converters, with which the two different voltages and frequencies are adjusted relative to each other, for example, using semiconductor switches (e.g. thyristors or gate turn-off thyristors—GTOs) in a direct conversion (AC/AC). Such direct converters exist, for instance, as so-called cyclo-converters or as so-called matrix converters (described, for example, in U.S. Pat. No. 5,594,636). In the case of a natural commutation, they can generate frequency components of a low-frequency that are undesired and difficult to eliminate, while in the case of forced commutation, they can entail large switching losses.
As an alternative, it is possible to ensure a voltage-adapted and frequency-adapted connection of a generator to an electrical power grid in the form of an indirect conversion. With such a conversion, first of all, a rectifier produces a direct current from the alternating current generated by the generator and, in an inverter, this direct current is subsequently matched to the voltage and frequency of the electrical power grid. Such controlled converters likewise make use of semiconductor switches (for instance, GTOs, insulated gate bipolar transistors—IGBTs, metal oxide semiconductor field-effect transistors—MOSFETs, or integrated gate commutated thyristors—IGCTs) and they can entail large switching losses at the switching frequencies employed.
Such a system and a mode of its operation is, for example, described in DE 103 30 473 A1. In this document, a method and a device for adapting the alternating current generated by a generator and the alternating voltage generated by a generator to a grid are proposed. The generator has at least one excitation coil and the power fed into the grid can be flexibly adapted while reducing switching losses in that a static frequency converter is employed for the adaptation between the generator and the grid, and in that, in order to control the power fed into the grid, means are provided with which, on the one hand, the strength of the excitation field generated by the at least one excitation coil is regulated and, on the other hand, the phase angle between the frequency converter voltage and the generator or grid voltage is appropriately controlled.